Gneiss // is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. It is foliated (composed of layers of sheet-like planar structures). The foliations are characterized by alternating darker and lighter colored bands, called "gneissic banding".
The etymology of the word gneiss is disputed. Some sources say it comes from the Middle High German verb gneist (to spark; so called because the rock glitters). It has occurred in English since at least 1757.1
Other sources claim the root to be an old Saxon mining term that seems to have meant decayed, rotten or possibly worthless material.
Gneissic rocks are usually medium- to coarse-foliated and are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. Depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, etc.
Gneiss appears to be striped in bands, called gneissic banding.2 The banding is developed under high temperature and pressure conditions.
The minerals are arranged into layers which appear to be bands, when the rock is broken and viewed from the side (in cross section).2 This is because of a different composition of each layer, or band, called compositional banding. The darker bands have relatively more mafic minerals (those containing relatively more magnesium - Mg, and iron Fe). The lighter bands contain relatively more felsic minerals (silicates, containing relatively more of the lighter elements, such as silicon, oxygen, aluminium, sodium, and potassium).
A common cause of the banding forms when the protolith (the original rock material that undergoes metamorphism) is subject to extreme shearing forces (a sliding force similar to when the top of a deck of cards is pushed in one direction, and the bottom of the deck in the other direction).2 These forces stretch out the rock like a plastic, and the original material is smeared out into sheets.
Some banding is caused by being formed from original rock material (protolith) that gets subject to extreme temperature and pressure was composed of alternativng beds of layers of sandstone (lighter) and shale (darker), which is metamorphosed into bands of quartzite and mica.2
Another cause of banding is "metamorphic differentiation", where chemical reactions separate different materials into different layers, a process not fully understood.2
Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Gneissose is used to describe rocks with properties similar to gneiss.
Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. These are amongst the oldest rocks in Europe and some of the oldest in the world, having been formed in the Precambrian "super-eon", up to 3 billion years ago. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree.3 These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma.4 The gneiss's delicate pink colours are exposed throughout the islands.5
Augen gneiss, from the German: Augen [ˈaʊɡən], meaning "eyes", is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands.
- Online Etymology Dictionary
- Essentials of Geology, 3rd Ed, Stephen Marshak
- Gillen (2003) page 44.
- McKirdy et al. (2007) page 95.
- Murray (1966) p. 2
- Blatt, Harvey and Robert J. Tracy (1996). Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed. Freeman, pp. 359–365. ISBN 0-7167-2438-3.
- Gillen, Con (2003). Geology and landscapes of Scotland. Harpenden. Terra Publishing. ISBN 1-903544-09-2.
- McKirdy, Alan Gordon, John & Crofts, Roger (2007). Land of Mountain and Flood: The Geology and Landforms of Scotland. Edinburgh. Birlinn. ISBN 978-1-84158-357-0.
- Murray, W.H. (1966). The Hebrides. London. Heinemann.
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